1. Field of Invention
The present invention relates to optical processing, and more particularly to the regeneration, reshaping and retiming of optical signals using analog microwave technology.
2. Description of Related Art
The telecommunications industry uses two types of signal processingxe2x80x94optical and electronic. Historically, optical processing has high transmission rates but has had very limited switching capabilities, whereas electronic processing has allowed for complex switching features, but transmission rates, especially as compared to optical processing, that are relatively slow.
A typical optical switching element used in the telecommunications industry is called a crosspoint switch. For the more usual case where there are N inputs and N outputs, this is called is called an Nxc3x97N array. For the case when there are M inputs and N outputs, this is called an Mxc3x97N rectangular array.
A crosspoint switch based on optical technology is called a wavelength crossconnect WXC and consists of a multiplicity of mirrors. Light signals on input wavelengths xcex1,xcex2,xcex3, . . . ,xcexN is converted by the WXC to light signals on output wavelengths xcex1xe2x80x2,xcex2xe2x80x2,xcex3xe2x80x2, . . . ,xcexNxe2x80x2. FIG. 1 shows schematic diagram of an Nxc3x97N crosspoint switch using optical technology. At each node there is the possibility of a connection between the input rows and the ouptut columns.
In such a wavelength crosspoint switch, the multiplicity of mirrors requires a large number of movable parts, and correspondingly causes an increase in the complexity and cost.
The bit rate B of information flow in each optical stream at each wavelength can be any one of the standard values. For example, B=2.5, 10, and 40 Gbps, for OC48, OC 192 and OC 768, respectively. The general trend is for the higher bit rates.
In FIG. 1, the array size is drawn for N=6. However, the array size for a crossconnect application will typically be appreciably larger, being large enough to accommodate about 100 fibers in each cable and about 20 wavelengths in each fiber. A typical crossconnect switch can therefore have N of about 2,000 to best optimize the performance of the communication network. Since some of these inputs simply let some wavelengths through without modification, it is possible to reduce the size of this crossconnect array to an N of about 1000 in certain circumstances.
Different chips in the industry have different values of B and N. The optimal values of the data points result in the general shape of a hyperbola, as shown in FIG. 2. For individual chips, large values of B can be obtained at small values of N and vice versa. What is desired is a combination of large values of B and N as shown by the circle in the figure.
It is possible to use tiling to assemble a multiplicity of chips into a system. FIG. 3 shows an example of tiling of 9 individual chips to make a system. The system of 9 chips is shown within the bold line. All interconnections are made on a printed circuit board and carry the full bit-rate. For example 100 68xc3x9768 chips can be, arranged to form a larger array of 10*68xc3x9710*68=680xc3x97680. Tiling obviously requires appreciable cost, especially at the higher bit-rates and larger array sizes.
In order to prevent degradation to the optical signal, grooming is required. There are three components to grooming 1R, which consists of regeneration, 2R, which consists of regeneration and reshaping, and 3R, which consists of regeneration, reshaping and retiming. In general, 3R components are more costly than 2R components, which are more costly than 1R components.
An example of a 1R component is an optical crossconnect that includes an array of optical mirrors that move a light beam with certain wavelength from one spatial position to another. This neither reshapes or retimes the optical signal.
An example of 2R component is described in a paper by P. E. Green et al entitled xe2x80x9cWDM Protocol-Transparent Distance Extension Using R2 Remodulationxe2x80x9d, IEEE Journal on Selected Areas in Communications, Vol 14, No. 5, (1966). The shape of a pulse is degraded after passing through many kilometers of fiber and many switches and amplifiers. A slicing circuit is used to sample and determine the mid-height of the rising portion of each degraded pulse, and the output from the slicing circuit drives a modulator to create a new square-shaped pulse in its place. But a large number of these shaping circuits are required, one for each wavelength. In the above example, this involves 128 such circuits in parallel. Furthermore at bit rates xcx9c10 Gb/s it is very difficult to design the last decision/driver stage, corresponding to the conversion from electrical to optical signals. Accordingly, such 2R components have not been widely used.
Conventional crosspoint switching arrays involve digital switches at each of the array nodes, and these digital switches also inadvertently provide some 2R functionality. The problem of these digital switches is that they must operate at the full bit-rate, which is very difficult, especially for bit-rates of 10-40 Gbps. An example is a Triquint crosspoint array, rated at 2.5 Gbps for a 16xc3x9716 array. These recent components have a very small product of bit-rate B and array size N and as such have limited utility in replacing optical crosspoint arrays. The crosspoint arrays described in this invention are very different than conventional crosspoint switching arrays, in major part since the crosspoint arrays of the present invention use analog switches instead of digital switches, as described further hereinafter.
An example of a generalized 3R electronic switch is Alcatel""s optical add-drop multiplexer (OADM) as described in the Ramaswami article entitled xe2x80x9cOptical Networks: A Practical Perspectivexe2x80x9d Morgan Kaufmann Publishers, San Francisco, Calif. (1998). The optical signals are converted to digital electronic signals, which are reshaped and retimed using digital signal processing techniques. The electronic signals are then used to modulate an optical beam, thereby producing an improved 3R optical signal.
Present telecommunication systems primarily use 1R components and 3R components within the system on an as needed basis in order to ensure signal integrity. Thus, these 1R and 3R components are positioned in various locations in a network. Historically, that has led to the widespread usage of 3R components in many locations within a network. While this has ensured that signal integrity has been maintained, the cost of the 3R components has been quite high.
It is an object of the present invention to provide a hybrid 2R component, which involves local regeneration and reshaping of the signal waveform in the analog microwave domain.
It is a further object of the invention to provide a hybrid 2R component with a more general functionality, including full wavelength switching and add/drop capability.
In order to attain the above object of the present invention, among others, the present invention provides a 2R component that uses microwave waveguides as opposed to the usual methods employing light deflection or digital signal processing. A preferred embodiment of the present invention includes: (1) an optical crossconnect (OXC) which allows any combination of wavelengths to be added or dropped from an element to an optical network, (2) an optical add/drop multiplexer (OADM) which allows any wavelength to be converted to any other, (3) a microwave crossbar switch array optimized for this purpose, and (4) a means for reshaping the exiting signal from the microwave crosspoint switch array without regard to signal format and without need for complex and expensive digital signal processing. The minimal architecture for this functionality is shown to be three Nxc3x97N microwave crossbar switch arrays.
Accordingly, the present invention enables an improved 2R component, which has advantages that will become apparent, particularly in the ability to provide renegeneration and reshaping of the signal waveform using a combination of components that can be provided for at costs that are much less than costs for 3R components and other 2R components. As a result, in a telecommunications system, fewer 3R components and fewer more costly 2R components are necessary, and the overall cost of the network can be substantially reduced.